NUMERICAL-SIMULATION OF COLLAPSING VOLCANIC COLUMNS

A complex thermo-fluid dynamic model was employed to model collapsing volcanic columns. The two-phase flow model accounts for the mechanical and thermal nonequilibrium between the gas and solid particles. The gas phase involves water vapor and air, and the solids phase involves only one particle size class. The particle collisions, which produce particle viscosity and pressure, were modeled by a kinetic theory model in terms of the granular temperature, whereas the gas phase turbulence was modeled by a turbulent subgrid scale model. The partial differential equations of conservation of mass, linear momentum, energy, and granular temperature were numerically solved for an axisymmetric flow configuration with different vent diameters and two-phase flow conditions. The numerical solutions involved different grid sizes and computational domains in order to assess the adequacy of the model and computational procedure. The results from simulations of collapsing volcanic columns show how after an initial period of fountain building the columns collapse and build radially spreading pyroclastic flows and inward moving column material which is recycled by the columns. For a low-height collapsing column it was found that the fountain reaches a steady state height, whereas for columns with collapsing heights of several kilometers and fine computational grids the fountain heights vary cyclically with periods which are influenced by the dynamics of material recirculation into the columns. The radially spreading pyroclastic flows of the collapsed columns were found to develop convective instabilities whereby rising clouds of gas and particles are developed on the top of the flows. In very large scale volcanic eruptions the numerical results predicted multiple rising clouds on very thick pyroclastic flows. The results from simulations were shown to be consistent with simple column modeling approaches, laboratory experiments, and field observations.